A goal of the Long Term Evolution (LTE) program of the Third Generation Partnership Project (3GPP) is to bring new technology, network architecture, configurations and applications and services to wireless networks in order to provide improved spectral efficiency, reduced latency, faster user experiences and richer applications and services with less cost. LTE's aim is to create an Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
In an LTE compliant network, discontinuous reception (DRX) operation is used by a wireless transmit/receive unit (WTRU) to save power. DRX allows the WTRU to sleep during regular intervals and wake up at specific time instances to verify if the network has data for it.
FIG. 1 shows a typical protocol stack architecture for an LTE network in accordance with the prior art. The system may include a WTRU 102, an e Node-B (eNB) 104 and an access gateway (aGW) 106. A non access stratum (NAS) protocol 108 and a packet data convergence protocol 110 (PDCP) may reside in the WTRU 102 and the aGW 106 to allow for communication between the devices. A radio resource control (RRC) protocol 112, a radio link control (RLC) protocol 114, a medium access control (MAC) protocol 116 and a physical layer (PHY) 118 may reside in both the WTRU 102 and the eNB 104 to allow for communications between those devices.
The RRC protocol 112 may operate in two states: RRC_IDLE and RRC_CONNECTED. While in RRC_IDLE state the WTRU DRX cycle is configured by signaling over the NAS protocol 108. This state includes system information broadcasts, paging, and cell resection mobility. A WTRU in RRC_IDLE state preferably is allocated an ID number that identifies the WTRU in a tracking area. No RRC protocol context is stored in an eNB.
In the RRC_CONNECTED state, the WTRU may make a connection with an E-UTRAN. The E-UTRAN knows the cell to which the WTRU belongs to so that the network can transmit and receive data to/from the WTRU. In the RRC_CONNECTED state, the network controls mobility (handover) and the WTRU conducts neighbor cell measurements. Furthermore, at the RLC/MAC level, a WTRU can transmit data to, and receive data from, the network and monitors a control signaling channel for a shared data channel to see if any transmission over the shared data channel has been allocated to the WTRU. The WTRU also reports channel quality information and feedback information to the eNB. A DRX/discontinuous transmission (DTX) period can be configured according to WTRU activity level for power saving and efficient resource utilization. This is typically under control of the eNB.
The NAS protocol 108 may operate in an LTE_DETACHED state, in which there is no RRC entity. The NAS protocol 108 may also operate in an LTE_IDLE state. Also, the NAS protocol 108 may operate in an RRC_IDLE state, while in LTE_DETACHED state, during which some information may be stored in the WTRU and in the network, such as IP addresses, security associations, WTRU capability information and radio bearers. Decisions regarding state transitions are typically decided in the eNB or the aGW.
The NAS protocol 108 may also operate in an LTE_ACTIVE state, which includes an RRC_CONNECTED state. In this state, state transitions are typically decided in the eNB or the aGW.
DRX may be activated in LTE_ACTIVE state, which corresponds to the RRC_CONNECTED state. Some of the services that would run in the LTE_ACTIVE state are those services generating small packets on a regular basis, such as VoIP. Also, those services generating delay insensitive bulk packets on an infrequent basis, such as FTP, may run in the LTE_ACTIVE, as well as those services generating small packets on a rare basis, such as presence service.
Based on the characteristics of the aforementioned services, data transmission/reception may be performed during DRX operation without RRC signaling. Also, a DRX cycle length should be long enough for battery power savings. Furthermore, the amount of data transmitted within a DRX cycle should be variable from cycle by cycle. For example, DRX for FTP service may allow an increase in the amount of data for each DRX cycle.
FIG. 2 shows a DRX signal structure 200 in accordance with the prior art. An active period 202 is the period during when a WTRU's transmitter/receiver is turned on and a sleep period 204 is the period during when a WTRU's transmitter/receiver is turned off. A DRX cycle length 206 is the time distance between consecutive active period start positions.
The DRX cycle length 206 may be determined by the network, considering the quality of service (QoS) requirements of a service activated in the WTRU. Active period start positions should be unambiguously identified by both the WTRU and the eNB.
At an active period start position, the WTRU may monitor an L1/L2 control channel during a predefined time interval to see whether there is incoming data. A length of the active period 202 may be variable, depending on the amount of data to be transmitted during the DRX cycle 206. An end position of active period 202 may be explicitly signaled by the eNB or implicitly assumed after inactivity of the predefined time interval. Uplink data transmission can be initiated anytime during the sleep period 204. Active period uplink data transmission may end when the uplink transmission is completed.
FIG. 3 is a signal diagram of a two layer DRX signaling scheme 300 in accordance with the prior art. The two layer method may be used to support flexible DRX and includes splitting the DRX signals into high level and low level. Referring to FIG. 3, a high level DRX signal 302 is controlled by the RRC. The high level DRX interval 306 depends upon the basic flow requirements of the connection, for example, voice over IP, web browsing, and the like. The high level DRX interval 306 is preferably determined by the RRC in the eNB and is signaled to the WTRU using RRC control signaling.
A low level DRX signal 304 is signaled by the MAC layer. A low level DRX interval 308 is flexible and may support fast changes in the DRX interval. A MAC header may carry information regarding low level settings.
Dependence between the high level DRX 302 and low level DRX 304 should be at a minimum because the high level DRX interval 306 can be used as fallback DRX interval in case of any errors occur applying the lower level DRX interval 308. The network and the WTRU preferably are synchronized with the high layer DRX interval 306.
The relatively long high level DRX interval 306 is beneficial for WTRU power savings, but limits downlink (DL) scheduling flexibility and throughput. If there is a significant amount of data buffered in an eNB or WTRU transmission buffer, it may be beneficial to change the short low level DRX interval 308 for a period of time suitable for the transmission of the buffered data. After the data transmission, the WTRU and the eNB could resume the high level DRX interval 302.
As shown in Table 1, DRX may be split between regular signals and interim signals.
TABLE 1Active mode DRX control signalingRRCMACRegularXDRXcontrolInterimXDRXcontrol
Signaling DRX in the RRC is based on the regularity of the basic connection requirements and may result in a regular DRX signal ensuring the requirements of the connection. Regular DRX is determined in the eNB. A WTRU should know, through RRC signaling, to apply regular DRX. In other words, when a WTRU enters active mode, one of the RRC parameters delivered to the WTRU will be the regular DRX parameters to be applied. While in active mode the eNB can change, at any point in time and through RRC signaling, the regular DRX parameters used by the WTRU.
FIG. 4 shows RRC signaling for regular DRX 400 in accordance with the prior art. An eNB 406 transmits an RRC signal 404 to a WTRU 402. The RRC signal 404 includes a regular DRX request. The WTRU 402 responds to the eNB 406 with an RRC signal 408 indicating that the WTRU received the regular DRX request.
MAC layer DRX may be able to handle fast and irregular changes such as, for example, an instantaneous increase of data throughput. The MAC layer interim DRX may be temporary. Interim DRX settings preferably are determined in the eNB. A WTRU acquires information regarding which interim DRX parameters to apply through MAC signaling. MAC signaling from the eNB to the WTRU may include interim DRX information. The WTRU may apply the interim DRX according to network instructions. Applying interim DRX does not affect the regular DRX interval. When a WTRU no longer applies interim DRX it will resume regular DRX.
FIG. 5 shows MAC signaling 500 for regular DRX in accordance with the prior art. An e Node-B 506 transmits a MAC signal 504 to a WTRU 502. The WTRU 502 responds to the eNB 506 with a hybrid automatic retransmit request (HARQ) process 508.